In today's catastrophic risks and resilience seminar, perhaps the most disturbing presentation was by J. Storrs (Josh) Hall, who gave a talk on “The Weather Machine: Nano-enabled Climate Control for the Earth.”

Josh offered a simple proposition: once molecular manufacturing is achieved, it should not be difficult to create a design so a nanofactory can produce a tiny transparent balloon fitted with GPS and radio (for sending data and receiving instructions) and a simple set of thrusters to maintain location and to control altitude.

So far, so good. But there are a few additions to this balloon that make its impacts pretty wild: first, it includes a mirror to be used either for reflecting sunlight back into space or directing it to a solar energy collector on earth; second, the mirror can be turned as instructed; and third, because the balloon is made by a nanofactory, as many of them as desired can produced and put into operation in a very short time.